<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text" id="num1">
    <select id="fh">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
        <option value="%">%</option>
    </select>
    <input type="text" id="num2">
    <input type="button" value="=" id="btn">
    <input type="text" readonly is="result" id = "res">
</body>
<script>

    let onum1 = document.querySelector('#num1')
    let ofh = document.querySelector('#fh')
    let onum2 = document.querySelector('#num2')
    let obtn = document.querySelector('#btn')
    let ores = document.querySelector('#res')
    
    var f=0;
  obtn.onclick = function(){
   switch(f){
           case "+"
           sun = onum1+onum2;
           
   }

    }
</script>
</html>